The present invention relates to conditioning of a polishing pad employed in chemical mechanical polishing (CMP). More particularly, the present invention relates to an apparatus and method for concurrent pad conditioning and wafer buffing in a CMP tool.
Chemical mechanical polishing (sometimes referred to as "CMP") typically involves mounting a semiconductor wafer on a holder and rotating the wafer face against a polishing pad mounted on a platen, which in turn is rotating or moving linearly or orbitally. A slurry containing a chemical that chemically interacts with the facing wafer layer and an abrasive that physically removes that layer is flowed between the wafer and the polishing pad or on the pad near the wafer. In integrated circuit (IC) wafer fabrication, this technique is commonly applied to planarize various wafer layers such as dielectric layers, metallization layers, etc.
FIG. 1 shows some major components of a chemical mechanical polishing (CMP) apparatus. Examples of such apparatuses include the AvantGaard 676 or 776, commercially available from Integrated Processing Equipment Corporation (IPEC) of Phoenix, Arizona, and described in IPEC Bulletins #4500-104621 and #4500-104660 (1997), which are incorporated herein by reference for all purposes. CMP apparatus 100 includes a wafer carrier 128 that is fitted with an air chamber 126 (shown in phantom lines), which is designed to secure a wafer 124 by vacuum to wafer carrier 128 during wafer loading typically before CMP is to commence. During CMP, however, wafer 124 is bound by "wear rings" (not shown to simplify illustration) within wafer carrier 128 such that a wafer surface that is to be polished contacts a polishing pad 102. During CMP, the polishing pad 102 orbits while the wafer 124 rotates.
A conventional polishing pad 102 for use with an apparatus such as illustrated in FIG. 1 includes a plurality of slurry injection holes 120, and adheres to a flexible pad backing 104 which includes a plurality of pad backing holes 118 aligned with the slurry injection holes 120. A slurry mesh 106, typically in the form of a screen-like structure, is positioned below the pad backing 104. An air bladder 108 capable of inflating or deflating is disposed between a plumbing reservoir 110 and the slurry mesh 106. The air bladder 108 pressurizes to apply the polishing force. A co-axial shaft 112, through which a slurry inlet 114 (shown by phantom lines) is provided to deliver slurry through the plumbing reservoir 110 and the air bladder 108 to the slurry mesh 106, is attached to the bottom of plumbing reservoir 110. Slurry is delivered to the system by an external low pressure pump, and is distributed on the polishing pad surface by centripetal force, the polishing action, and slurry pressure distribution on the pad 102. The polishing pad 102 may also be provided with grooves or perforations (not shown) for slurry distribution and improved pad-wafer contact.
Unfortunately after polishing on the same polishing pad over a period of time, the polishing pad suffers from "pad glazing." As is well known in the art, pad glazing results when the particles eroded from the wafer surface along with the abrasives in the slurry tend to glaze or accumulate over the polishing pad. A glazed layer on the polishing pad typically forms atop eroded wafer and slurry particles that are embedded in the porosity or fibers of the polishing pad. Pad glazing is particularly pronounced during planarization of an oxide layer such as silicon dioxide layer (hereinafter referred to as "oxide CMP"). By way of example, during oxide CMP, eroded silicon dioxide particulate residue accumulates along with the abrasive particles from the slurry to form a glaze on the polishing pad. Pad glazing is undesirable because it reduces the polishing rate of the wafer surface and produces a non-uniformly polished wafer surface. The non-uniformity results because glazed layers are often unevenly distributed over a polishing pad surface.
One way of achieving and maintaining a high and stable polishing rate is by conditioning the polishing pad (the process of conditioning a polishing pad is hereinafter referred to as "pad conditioning") on a regular basis, e.g., either every time after a wafer has been polished or simultaneously during wafer CMP. During pad conditioning, a conditioning arm or an abrasive disk is typically contacted with a polishing pad, which may be rotating or in an orbital state.
FIG. 2A shows a top view of some significant components of a conditioning sub-assembly 200, which may be integrated into a CMP apparatus such as the IPEC 676. Conditioning sub-assembly 200 includes a polishing pad 202 and a conditioning arm 204 that is disposed above polishing pad 202 and capable of pivoting about a pivoting point 206. Conditioning arm 204, as shown in FIG. 2A, is typically longer in length than a diameter of the polishing pad. For illustration purposes, FIG. 2B shows a bottom view of conditioning arm 204 of FIG. 2A. The bottom surface of conditioning arm 204 includes a plurality of diamond abrasive particles 208, which are substantially uniformly arranged on the conditioning arm such that if conditioning arm 204 contacts polishing pad 202, abrasive particles 208 engage with a substantial portion of the polishing pad.
Before conditioning sub-assembly 200 of FIG. 2A begins conditioning of polishing pad 202, conditioning arm 204 is lowered automatically to contact a polishing pad 202, which may be rotating or in orbital state. A pneumatic cylinder (not shown to simplify illustration) may then apply a downward force on conditioning arm 204 such that abrasive particles 208 contact and engage with a substantial portion of polishing pad 202. During pad conditioning, conditioning arm 204 pivots on pivoting end 206 and sweeps back and forth across polishing pad 202 like a "windshield wiper blade" from a first position 204' (shown by dashed lines) at one end of the polishing pad to a second position 204.DELTA. (shown by dashed lines) at the other end of the polishing pad. This mechanical action of conditioning arm 204 allows abrasive particles 208 to break up and remove the glazed or accumulated particles coated on the polishing pad surface.
At the conclusion of some CMP procedures, a fine polishing, also referred to as buffing, is often performed on the wafer in order to produce the smoothest possible wafer surface. Buffing typically uses a relatively soft pad formed, for example, from polyurethane impregnated felt. An example is the Polytex.TM. pad available from Rodel Corp. of Newark, Del. Buffing also typically uses deionised water or may be assisted by a conventional oxide slurry.
Unfortunately, currently used pad conditioning and wafer buffing systems reduce the efficiency of CMP operations. FIG. 3 is a simplified top view of a typical multi-station CMP apparatus, such as the IPEC 676 or 776, described previously. The CMP apparatus 300 has four polishing stations 302, 304, 306 and 308, each with a polishing pad 310 and the other associated features described with reference to FIG. 1 (not shown in this view to simplify illustration). As shown in FIG. 3, the apparatus 300 also includes two conditioning sub-assemblies 320 and 322, such as described with reference to FIGS. 2A and 2B, each of which service two polishing stations. For example, as shown in FIG. 3, conditioning sub-assembly 320 services polishing stations 302 and 304. Each conditioning sub-assembly 320 includes a conditioning arm 324 that may be swung out above a polishing pad polishing pad 202 by pivoting about a pivoting point 326.
With conventional CMP techniques, one of the four stations on the CMP apparatus is typically dedicated to buffing, or a separate buffing station must be provided in addition to the polishing stations. This reduces the number of polishing stations available on the apparatus, or requires that the wafer be moved to a separate buffing station for buffing, both of which result in a significant reduction in the through-put capacity of the machine. Moreover, since a polishing pad must be conditioned following each wafer polishing, that pad is unavailable to receive another wafer for polishing until conditioning is complete. This is a further impediment to CMP efficiency.
Therefore, an improved apparatus and process for pad conditioning and wafer buffing that improves the efficiency of the CMP process would be desirable.